The extra water required to assist in the crystallization process when forming a hydrate (mortar, cement, concrete, plaster, etc.). When the hydrate gives up this excess water, at ambient temperatures, the result is a surface deposit known as efflorescence or staining.
Water in chemical combination with a crystal, necessary for the maintenance of crystalline properties, but capable of being removed with sufficient heat. Water in calcium sulfate dihydrate f was(gypsum) or calcium sulfate hemihydrate. Also see Combined Water.
Water of crystallization (alt. Br.E. water of crystallisation) is water that occurs in crystals but is not covalently bonded to a host molecule or ion. The term is archaic and predates modern structural inorganic chemistry, coming from an era when the relationships between stoichiometry and structure were poorly understood.